| Phylogenetic Relationships among DNA's of wheat, rye
and Agropyron J. DVORAK and E. SCHELTGEN Departments of Crop Science and Bacteriology, University of Saskatchewan, Saskatoon, Canada There is much evidence to indicate that the genera Triticum, Secale and Agropyron are closely related. Compensation for replaced wheat chromosomes in alien disomic substitutions has been demonstrated for a number of rye and Agropyron chromosomes, which indicates that homoeologous chromosomes of these species must carry similar genes. On the other hand, the fact that little or no pairing occurs between wheat and Agropyron or rye homoeologues suggests considerable dissimilarity among these chromosomes. Numerous structural rearrangements of chromosomes may have occurred in these genomes during their evolution. Indeed, it has been shown that the S. cereale genome differs from the more primitive genome of S. montanum by a minimum of three reciprocal translocations (RILEY, 1955) and diploid A. elongatum differs from Ae. squarrosa by a minimum of one reciprocal translocation (DVORAK, 1971). However, the well-expressed homoelogy among Triticum, Aegilops, Agropyron and Secale chromosomes suggests that the fixation of translocations has been relatively rare. Studies of pairing between specific wheat and Aegilops or Agropyron telocentrics in polyhaploids involving Ae. speltoides have revealed that chromosome pairing is usually restricted to one homoeologous group (JOHNSON and KIMER, 1967; KIMBER, 1968; ATHWAL and KIMBER, 1972; DVORAK, 1972a), which makes the presence of numerous translocations in these chromosomes highly improbable. However, as KIMBER (1968) and ATHWAL and KIMBER (1972) were able to demonstrate for Ae. umbellulata chromosomes, inversions would preserve the integrity of homoeologous groups and yet structurally differentiate homoeologues. The high incidence of inversions constituting intra- and inter-speciflc chromosome polymorphism within the genus Drosophila (STONE et al., 1960; WASSERMAN, 1960; BocK, 1971) indicates that inversions may be the most significant type of chromosome rearrangement during evolution. The question of whether the low pairing affinity among homoeologous chromosomes is due to the accumulation of inversions in genomes of Triticinae or to some other cause cannot be resolved for lack of evidence. While the mechanism of chromosome pairing is still obscure, there is some justification for a hypothesis that pairing is initiated in specific sites along chromosomes. It was shown in corn that segments either close to a telomere or proximal to a centromere have minor importance in the initiation of homologous pairing (BURNHAM et al., 1972). COMINGS and RIGGS (1971) proposed that nucleotide sequences in specific sites along chromosomes may play an essential role in the recognition of homologous chromosomes. DVORAK (1972b) suggested that the diversification of such sequences during evolution may be responsible for low pairing affinity between homoeologues. From this point of view, it appears worthwhile to obtain information on the extent of similarity in DNA's of representative species in the Triticinae. |
| Copied from the Proceedings of the 4th Wheat Genetics Symposium by the kind permission of the Editors and Organizing Committee. |
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